VW Lupo Diesel Performance Issues & Rough Running Fix

I had a very similar issue with my VW Polo diesel from 2003. As someone with moderate experience working on cars, I initially thought it was related to the mass airflow sensor too. The symptoms matched exactly what you describe, poor acceleration, rough idle, and the check engine light constantly illuminating. After some initial diagnosis, the workshop found that one of the fuel injection valves was defective, causing uneven fuel delivery to the engine. The complete repair included: Testing all injection components, Replacing the faulty injector, Recalibrating the fuel injection system, Clearing fault codes Total cost was 515€ including parts and labor. Since the repair, the engine runs smoothly again with no power issues or warning lights. To help diagnose your case more accurately, could you provide: Current mileage, Service history details, Whether the check engine light shows any specific error codes, If you notice black smoke from the exhaust, When the symptoms first appeared The catalytic converter and other components might also be affected if this issue has been present for a while, so these details would help determine if additional checks are needed.

Last service was done at 125000km, about 2 months ago. After reading through the responses, I had the error codes read at a local shop. They showed P0401 which points to the EGR system. The rich mixture and reduced power make sense now. Looking at the repair cost estimate of 515€ for the injection system seems reasonable, but Im hoping the EGR cleaning might be a simpler fix. The mass airflow sensor readings appear normal according to the diagnostic scan. Has anyone dealt specifically with P0401 codes on these engines? Our symptoms sound very similar though, especially the performance issues and how the throttle body responds. Going to have the intake manifold checked as well since they mentioned possible carbon buildup affecting the airflow.

Thanks for providing those details about the P0401 code. Actually, when I faced similar symptoms on my VW Polo, I initially had the EGR cleaned, but the problem persisted. The rich mixture issue turned out to be more complex than just the EGR system. In my case, we discovered carbon deposits in the intake manifold were restricting proper airflow, working together with the faulty fuel injection to create those symptoms. The spark plugs were also heavily fouled due to the rich fuel mixture running for some time. Looking at your situation, I would recommend: Getting the intake manifold thoroughly cleaned, Having all fuel injectors tested, not just cleaned, Checking for vacuum leaks around the intake system, Testing the O2 sensors as they affect fuel trim While the EGR might be part of the problem, my experience showed these symptoms often point to multiple related issues. The check engine light may show one code, but the rich mixture and performance problems suggest checking the entire fuel delivery system. If you see any improvements after the EGR cleaning, consider having the intake system inspected before investing in injector repairs. The carbon buildup you mentioned could be the primary cause of your problems.

Thanks for the suggestions regarding the P0401 code and sharing your experience. I checked with my regular workshop and they confirmed the EGR valve needs cleaning first. The air-fuel ratio being off makes sense given the symptoms. The intake manifold inspection sounds like a smart preventive step. The check engine light stays on even after resetting, which matches what you described. Really helpful to know about the carbon deposit issue you encountered, will definitely have them check that while they clean the EGR. My quotes so far: EGR cleaning and testing: 180€, Full intake system inspection: 95€, Diagnostic check of oxygen sensors: 60€ Planning to start with these before considering the more expensive injector work. The oxygen sensor testing seems worthwhile given how it impacts the fuel mixture. At least the mass airflow sensor readings checking out normal rules out one potential problem. Will update once I get the EGR cleaned and see if the engine performance improves. Thanks for helping me avoid replacing parts unnecessarily.